Sunflower Electric Power's two new 700-megawatt generators near Holcomb, Kan., were supposed to be part of a wave of new coal-fired power plants heralding coal's comeback as America's fuel choice for cheap energy. They were to join at least two dozen other coal-power projects already under construction in the US.

But that was before last week. That's when a Kansas regulator pulled the plug on Sunflower's proposal, citing concerns that the plants' emissions would contribute to global warming.

It's the latest hiccup – perhaps even a major interruption – in plans for a big resurgence in coal power, which already provides about half of US electricity. Among other concerns that are unsettling to coal advocates: soaring construction costs due to international competition for building materials, and worry among utilities that Congress will craft climate legislation that attaches a high price tag to greenhouse-gas emissions.

Just nine months ago, the federal government listed more than 150 coal-power plants as "in development." Since then, at least 16 have been canceled, and many others have been put on hold, according to data from the US Department of Energy (DOE). Sunflower's plants appear to be the 17th and 18th coal plants to get bumped this year – but not the first to get bumped because of regulators' climate concerns.

At least two plants in Florida were halted this summer amid climate concerns. Regulators in Oklahoma, also citing fears that coal power's costs would increase with federal climate legislation, last month rejected a proposal for a $1.8 billion coal-fired plant by a subsidiary of American Electric Power, the big Ohio-based utility.

While Sunflower officials vowed to fight the Kansas order, environmentalists were cheering the regulators.

"This is a big deal," says David Hawkins, climate director for the Natural Resources Defense Council. "We can be sure that most if not all proposed plants will face intense opposition from community and national groups and increased scrutiny by regulators and investors."

Overall, coal-fired power plants in the construction pipeline fell to 121 from 151 in May, according to a new report by the National Energy Technology Laboratory (NETL), a division of the DOE. Eight were removed from the NETL list because they were canceled, nine because they were put on hold, and three for unspecified reasons. Another 10 power plants were removed from the in-the-pipeline list because they were completed.

Not listed at all were eight power plants planned for Texas but canceled in February.

To many in the industry, the drop in numbers is not a big a surprise.

"Any time you see a big new push for construction, you have a lot of plants that just don't make it through to completion," says Ed Legg, spokesman for the Edison Electric Institute (EEI), a trade group that represents investor-owned utilities. "It could be carbon awareness, you're certainly seeing that. But it's not really the main thing. Any time factors like cost change as much as they have, that can put the brakes on."

Construction abroad – notably in China and India, which are building power plants and factories at a fast clip – has led to a shortage of steel, concrete, and engineering expertise. That drives up costs, changing the construction calculus. Costs for the average new US coal-fired plant have risen by about 40 percent since 2000, according to a new EEI study.

The setbacks for coal plants worry some experts. "These plants are clearly not coming in as quickly as announced," says Ken Kern, director of the office of systems, analyses, and planning at NETL. "The question we have now is: Are these coal plants going to arrive in time to meet the nation's growth in power demand?"

Forty-five new coal plants are currently listed by NETL as "progressing," meaning they are under construction or near construction, or have received some permits. Projects in late stages of development are more likely than not to be completed.

That number is not high enough to comfort Mr. Kern, who worries that the electric grid will not be as reliable as it needs to be because it will lack the capacity to meet peak demand. The National Energy Reliability Council raised similar concerns in a report this week, citing a "high degree of dependence on natural gas" in Florida, Texas, the Northeast, and southern California that, it said, could lead to power failures.

EEI's Mr. Legg describes himself as "continuing to be bullish" about the prospects for coal power, because he thinks it's the fuel most likely to supply future demand for electricity, which is forecast to rise 40 percent by 2030.

Utilities will need to add about 139 billion watts of coal-fired generating capacity by 2030 to meet demand and replace old plants being retired, according to the DOE's Energy Information Administration. That's some 278 coal-power plants at 500 megawatts each – about 12 per year for the next 23 years.

That would be an environmental disaster if the new plants use conventional pulverized coal-burning technology rather than advanced coal gasification that allows for capture and storage of greenhouse gases, says John Thompson, director of the Coal Transition Project of Clean Air Task Force, a Boston-based environmental group. The US needs to slash carbon emissions by 80 percent below 1990 levels by 2050 to avoid the worst effects of global warming, he says.

"We should catch our breaths – and do the right thing," he says, referring to a switch to gasification and carbon-capture technology.

"There's no question the local concerns are ratcheted up with at least the perception that these facilities have on global climate," says James Stanton, director of NERC compliance at ICF International, an energy consulting firm in Fairfax, Va. "The NIMBY stuff has always been there, but now it's not just 'not in my backyard;' it's 'not on my planet,' too."